. Although over one-third of all breast cancers are diagnosed among women aged 65-79 years, little epidemiologic research has specifically focussed on breast cancer in women of this age. Women of this age also commonly suffer from hypertension or coronary disease, and calcium channel blockers (CCBs) are often used to treat either of these conditions. Two recent studies, yet to be published, of women aged 65 and older, suggest that women who use CCBs may be at an increased risk of breast cancer. This suspected association is given biological plausibility by the observation that pharmacological blockade of the calcium channels can inhibit apoptosis (programmed cell death), the process whereby organisms eliminate unwanted cells (e.g., preneoplastic, initiated, damaged, excessive). In this sense, CCBs may be cancer promoters. The application proposes a case-control study of 1,000 women, aged 65-79, who reside in King County, Washington, who are on the Health Care Financing Administration (HCFA) tapes, and who are diagnosed with their first invasive breast cancer during the time period of January 1, 1997 through December 31, 1999. The personal interview responses of these women about drug use and other known risk factors for breast cancer will be compared to a control group of 1,000 women without breast cancer who will be identified through the HCFA tapes. Based on the preliminary studies and the evidence from in vitro studies on cells, it is posited that the use of CCBs increases the risk of breast cancer in older women. The plan is to assess whether calcium antagonists used in the treatment of hypertension and cardiovascular diseases promote breast cancer in women aged 65-79 years, and whether any one type of calcium channel blocker is more related to breast cancer than the other types. Since no studies have assessed present or past use of combined estrogen-progestin therapy in a large group of women this age, and the concurrent use of hormone replacement therapy (HRT) by these women may affect the proposed estimates of risk associated with CCBs, data will also be collected and analyzed on other drugs (viz., cimetidine, anti-depressants, lipid lowering drugs) frequently used by women in this age group.